A comma separated list of weekdays (MO,TU..). Each weekday can have be preceded by a positive or negative number (1MO - 1st monday of the month/year, -1MO - last monday of the month/year).

bymonthday
optional
string

A comma separated list of month days (1-31). Each month day can have a plus(+) or minus(-) prefix.

bysetpos
optional
string

A comma separater list of the nth occurance within bymonthday. If you wish to set a repeat to occur on the last day of the month, set frequency to monthly, bymonthday to 28,29,30,31 and bysetpos to -1.